1. Field of the Invention
This invention relates to digital signal processing apparatus.
2. Description of the Prior Art
It is of course well known to produce a digital picture signal comprising a plurality of digital words representing respective pixels (picture elements) which, when arranged in a two-dimensional array, make up the picture. Such a signal may, for example, be a digital video signal obtained by sampling an analog video signal and digitizing the samples, by pulse code modulation, to form digital words, for instance 8-bit words. There are, however, other applications in which digital picture signals may be formed, for example in digital storage of image information and character recognition.
It is also well known to store digital picture signals. For example, digital video effects (DVE) equipment will often store one or more fields or frames of a digital video signal in order that the picture may be processed or manipulated to achieve a desired effect.
In modern picture processing techniques, and in particular (but not exclusively) in DVE equipment, access to a two-dimensional (2-D) portion of a stored digital picture signal often is required. The portion usually is of a fixed size and may be located at any position within the picture. For instance, it may be desired to process an array of pixels in a fixed function digital filter, in which an array of pixels are combined with predetermined weightings to produce an output pixel, or in an interpolating digital filter in which an array of pixels surrounding an output pixel position not coincident with the position of a stored pixel are combined with predetermined weightings to produce an interpolated output pixel to sub-pixel accuracy.
Naturally, whatever the application, it is desirable that access to the stored digital words representing the pixels be achieved in as fast a manner as possible, in order to maximize the processing speed. To this end, it is highly desirable that the stored words representing the desired pixels be accessible in parallel. Such parallel access not only enables fast processing, but enables the processing to be achieved in a pipelined manner in that sets of words can be fed into a filter (for example) in parallel whereby an output from the filter will arrive after a predetermined delay.
Our above-cited copending U.S. patent application Ser. No. 07/482,050 discloses a method of obtaining access to a two-dimensional portion of a digital picture signal. The method enables parallel access to be achieved. However, it poses a difficulty in the construction of a digital filter. Specifically, as described in more detail below, it involves storage of digital words in a plurality of memory devices in such a way that there is a plurality of possible relationships between the words of a set of words read from respective different ones of the memory devices and the respective memory devices in which they are stored.
An object of the present invention is to enable the above-mentioned difficulty to be overcome, that is to solve the problem of constructing a digital filter that will take account of the fact that there is a plurality of possible relationships between the stored words and the devices in which they are stored.